WCXX

I'm not sure how easy it is for him to maintain this attitude though, as a few minutes later a middle-aged woman bears down on our table brandishing a shiny new hardback fresh from the bookshop next door, which she requests him to sign for her daughter. Does this happen often? "It happens quite a lot, yeah," he says, with a look that can only be described as sheepish.

He has not achieved this level of popularity by shying away from the darker side of the human mind, in his portrayal of his heroes as well as his villains. Nesbø's latest novel, Police, which boasts the customary ability to render attempts at work and sleep futile until it is finished, sees the return for the 10th time of his brilliant but wayward detective Harry Hole, a man so tortured by his inner demons that you feel he could have been the sitter for Munch's painting. Here Harry finds himself contemplating infidelity, indulging in violent rape fantasies and questioning whether he can regard himself as morally superior to the serial killer he is hunting.

"I'm not sure the fantasies of, let's say, criminals and non-criminals are that different," says Nesbø. "I think that most people have fantasised about having their loved ones killed or raped, so that we can then justify our revenge fantasies.

"Most of us have a sort of internal censorship, and if there's something we shouldn't think about, like maybe having sex with your mother, you will think about it for a fraction of a second and then stop yourself. But as a writer what you should do is stop and roll with that for a moment. You think OK, if you had sex with your mother, what would you say in the morning over breakfast? And your readers can read about it without having to take the responsibility of that being their own thoughts."

Ever since he was a child and noticed that one of his fellow pupils took the trouble to bring tweezers to school in order to pull the legs and wings off flies, he has been interested in what makes people do evil things. "I thought, do the bad guys see themselves as bad guys or good guys? If I went inside their heads, would I see things totally differently, like a photo negative?"

He also took the time to wonder how the flies felt about it, he says. He now admits that this sort of empathetic curiosity – how does a man feel if he's been tied to a hot oven and the person torturing him keeps throwing water in his face to stop him from fainting? – has led him to overdo some of the more violent scenes in previous books.

Surprisingly, he confesses to having little interest in the Norwegian mass-murderer Anders Breivik. "Maybe it's because his actions were on such a big scale that you can't really relate them to everyday things… I'm actually more interested in someone who might push in the queue here and wondering, OK why did he do that, what kind of man is he? Then I can start making up a story about what his life is like."

He was more intrigued by the Norwegian people's reaction to Breivik's atrocities. "It was very touching that people refused to react with anger and violence and paranoia. It was reassuring to realise that you live in a society of mostly decent people.

"Then the Norwegian news media started to think about what the international media was saying about Norway, and we got self-conscious about the way we reacted. And so we started writing about our reaction to what happened as if we had won a gold medal in the Olympics or something, and that was when I made a couple of comments about how this was not a competition in dignity, and if it was, we just lost. But we can still look back with pride at the spontaneous reaction of the people I think."

The key to his success, Nesbø affirms once more, is to be interested in ordinary people rather than psychotic monsters. "The important thing is to live a life among your readers, have experiences they can relate to. But also I think it's good for writers if they are slightly crazy. That gives you that different view of life you need to be a good writer."

He admits then to being slightly crazy himself? He fixes me with that piercing gaze again. "I guess I am. Or I try to be."

Police, by Jo Nesbø, is published by Harvill Secker on September 12. To pre-order a copy from Telegraph Books for £16.99 plus £1.35 p&p, call 0844 871 1514 or visit books.telegraph.co.uk (rrp £18.99).

For dates and details of all Jo Nesbø's forthcoming UK and Ireland events, visit jonesbo.co.uk

For anyone entering our £5,000 Telegraph Harvill Secker crime writing competition, Nesbø has given two masterclasses about his Harry Hole thrillers. See telegraph.co.uk/crimecomp, where you can also find details of the competition. The closing date for entries is November 30.